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Aviation History
1928
1928 - 0669.PDF
JULY 19, 1928 57 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT .•. B = 2-54 ft. = 30-5 in. andD = 1-13 X 30-5 = 34-5 in. andL = 8 X 2-54 = 20-3 ft. Taking the vee angle of the planing bottom as 30°, the midship section can readily be drawn in to give a midsliip- area coefficient of 0 • 75, checking the area with the aid of the planimeter. Figs. 3 and 4 show the old and the new shapes of planing bottom. The flare on the bottom shown in Fig. 4 is reminis- cent of the bows of torpedo-boats and light cruisers. It accomplishes two useful purposes. It reduces the spray angle, lessening the height of the cone of water developed between the floats when tas3ring, and further, is found to add to stability in a longitudinal direction. The effect of a locally extended planing bottom on rolling is also of interest, in particular with large machines where the chine becomes the equivalent of a bilge keel, giving a steadying effect without seriously increasing water resistance. The next step is to draw in the keel line on side elevation, the after body keel line being a straight line at approximately 7° above the horizontal. The chine line aft is similarly a straight line curving down at the end station to meet the keel line. The depth of step necessary is dependent on the " getting- off " speed of the machine. As the " getting-off " speed decreases, a deeper step ia required. To determine approximately the depth of step divide the extreme breadth of the float by thirteen. The body sections of the forward portion of the float should be designed to give a centre of buoyancy situated forward of the step, to allow the machine to trim up by the nose and ride cleanly at anchor. The top of float is taken as horizontal over the entire length. For aerodynamic reasons, floats have been specially built for racing machines with a " cigar" nose.* It is considered, however, that the value of this shape for other than racing craft is outweighed by considerations of expendi- ture in manufacture, and from the service view-point it seems a debatable point whether the amount of reduction of drag is not offset by the probable encouragement given to the floats to nose under when pitching in rough water at anchor. It is assumed that the reader is familiar with the process of fairing-in the lines by the use of waterlines, half-breadths and buttocks. There is, unfortunately, no royal road or short-cut to the actual " fairing-in " of the lines. The body-sections have now to be drawn in to give the required volume, the load waterline determined and its shape laid off in plan to see how near it • comes to the desired coefficient of fineness. By using an angle for V-bottom of 30° at midships, varying to 40° at bow and 35° aft a minimum of trial and error will be involved to obtain the required results. (See Fig. 1.) The lines plan is generally drawn to a convenient scale in the drawing-office, but, following ship practice, the lines are later laid out full size in the shops, corrected for fairness, and a corrected list of offsets supplied to the drawing-office to which the detail design of parts can be accurately carried out. Before fairing-in the lines finally for model test it is advisable to try out the approximate metacentric height of the flotation system. This is done by using a coefficient for the moment of inertia of the waterplane. The track of the floats is assumed to be approximately fixed on the general arrangement drawing of the machine and likewise the position of centre of gravity of the machine is assumed known. The moment of inertia coefficient may be taken as 0-04. Then 0-04 LB'= Moment of inertia of one float about c.l. of float. Where L = Length of float. And B == Breadth of float. And l0 = moment of inertia about c.l. of machine = 1 of W.P. X J track2). (area Transverse metacentric height = — — B G."j" Where V = Displacement in lbs. And B G = Distance between centre of gravity of machine and C.B. of float. This is worked out and if considered satisfactory, a model of the floats with the given track is constructed on, say, a tenth scale for the purpose of tank tests which will determine the finally corrected lines which later will be laid down full- size for constructional purposes. It may be stated here that tank tests are indispensable. A set of float lines may look fair and meet all the estab- lished requirements, but tests in the tank will in ninety-nine cases out of a hundred indicate where improvements in cleanliness of running and in longitudinal stability can be effected. It has only to be recollected that naval architects with over a 100 years' accumulated data in the matter of ship form design, still consider tank tests of hull-shapes a necessity, to appreciate the position of the designer of seaplanes. For this reason, the method indicated of using coefficients to draft out the lines must not be regarded as other than a reasonably quick method of designing a float shape which is likely to call for a minimum of re-adjustment in proportions and contours. Model Tests The basis of all model tests is Froude's law of comparison. The law enables one to compare the resistances of— (1) Model with full-size float. (2) Floats of similar form. It takes into account eddy-making and wave-making resistances but not frictional, i.e., " skin " resistance, for which a correction is always made by naval architects in considera- tion of the disparity in length between the model and the full-size ship, since frictional resistance is expressed as :— /w\ /v R = / £ xSx -W. where R = resistance in lbs. S =, area of wetted surface in sq. ft. V = speed in knots. f = a coefficient depending on nature and length of the surface. W = density of salt water. Wo= density of fresh water. In using Froude's Law of comparison there are certain con- ditions which must be observed. (a) Speed. " In comparing similar vesse's, or vessels with models, the speed must be proportional to the square root of their linear dimensions." "^ ' The linear dimension is taken as the overall length of the float; for example, the speed at which the model would have to be run to give comparable results with a float 20 ft. long travelling at 30 knots would be arrived at aa follows :— Length of float = 20 ft, ,, model = 2 ft. V (speed of float) = 30 knots. Let v = speed of model in knots. Then - = A /2- =, A / 10v V 2 V • These floats proving highly satisfactory in all respects. V 30 . •. v = —~ = —~ =9-5 knots. •y/io v10 These speeds are referred to as " corresponding speeds." When running the model, the latter is loaded to the equivalent water-borne weight for each speed, these speed* ranging from about 10 knots to 50 knots for full-size machine. t See " Seaplane Stability Calculations," AIRCRAFT ENGINEER, Feb. 23and M»rch29, 1928. 614e
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